A demonstrator stands beside a mock drone near Fort McNair where President Obama spoke May 23 at the National Defense University in Washington. Obama gave a robust defense of the use of unmanned armed drones and said the threat of terrorism had shifted since 2001.

The two presidents since the 9/11 attacks – George W. Bush and Barack Obama – have spoken many times about the morality of their tactics against terrorists. The two differ widely on their approach, as do many Americans. Yet any counterterrorism strategy relies heavily on a national consensus in what is essentially a moral contest with Al Qaeda and like-minded groups.

On Thursday, President Obama tried once again in a speech to define the moral basis for his actions – in his use of predator drones even if they kill civilians, in trying to close Guantánamo Bay even if released detainees return to terror, in targeting American terrorists overseas even if that denies them constitutional rights.

As Mr. Bush did, Obama wrestles daily with the Gordian knot of legal, practical, and moral demands related to ending terrorist attacks. In this latest speech, he made a strong case for his approach, considering the difficult trade-offs in dealing with an enemy that is stateless and ruthless, and whose horizon for conflict seems endless.

“Our response to terrorism cannot depend on military or law enforcement alone,” he said in his speech at the National Defense University. “We need all elements of national power to win a battle of wills and ideas.”

Moral defensibility often wins wars. As British historian Richard Overy wrote about World War II, the Allies won in part by relying on “the language of liberation and resistance” against the Axis powers. John Kennedy rejected a Pearl Harbor-like sneak attack against Soviet missiles in Cuba in hopes of keeping the moral high ground. Such an approach helps unify people, wins over allies, and perhaps even helps demoralize an enemy by showcasing the bankruptcy of its moral universe.

Obama’s moral arguments for his antiterror tactics have several intended audiences – courts, Congress, the American people, and close allies like Britain. Most of all, he wants to win over Muslims who might lean toward jihad based on arguments that America’s actions are immoral. Not all will be convinced, but each president must keep making a moral case.

Obama, a former constitutional professor, at least makes his points simple. Take the drone strike that killed American citizen Anwar al-Awlaki, the Al Qaeda chief in Yemen: “His citizenship should no more serve as a shield than a sniper shooting down on an innocent crowd should be protected from a SWAT team.”

And as for civilians killed in drone strikes despite Obama’s “highest standards” to prevent such harm: “Let us remember that the terrorists we are after target civilians, and the death toll from their acts of terrorism against Muslims dwarfs any estimate of civilian casualties from drone strikes.”

And on the need to close Guantánamo: “Imagine a future – 10 years from now, or 20 years from now – when the United States of America is still holding people who have been charged with no crime on a piece of land that is not a part of our country. Look at the current situation, where we are force-feeding detainees who are holding a hunger strike. Is that who we are?”

Many in Congress, left and right, will offer practical reasons opposing many of his arguments. And Obama must still answer Muslims who decry his support for the antidemocratic and sometimes ruthless monarchies in Saudi Arabia and Bahrain.

Defending America’s security based on its values requires a constant – and democratic – struggle to define, hone, and implement those values. Of all the speeches by Obama and Bush to forge such a consensus, this latest one should bring more agreement than any other. It was meant to get the US off “a perpetual war-time footing” by narrowing the use of past tactics. A moral unity will help.